1. Field of the Invention
The present invention relates to a method for manufacturing a SIMOX wafer, and a SIMOX wafer manufactured thereby, and in particular, the present invention relates to a technique for satisfactorily maintaining surface flatness of the SIMOX wafer in which a BOX (buried oxide) layer is locally formed.
This application claims priority from Japanese Patent Application No. 2009-150966, filed on Jun. 25, 2009 and Japanese Patent Application No. 2010-142214, filed on Jun. 23, 2010, the contents of which are incorporated herein by reference.
2. Background Art
In an SOI (Silicon On Insulator) wafer, a silicon oxide film (referred to as “BOX layer”) is buried in a silicon single crystal wafer, and an upper layer of this BOX layer is covered by a silicon single crystal thin film (referred to as “SOI layer”). In the SOI wafer, a substrate portion and the SOI layer which is a device manufacturing layer can be electrically separate from each other. For this reason, satisfactory properties are attained in breakdown voltage, parasitic capacity, and radiation resistance capability without occurring the substrate bias effect. As a result, effects such as high speed, low power consumption, and soft error free are expected, and a variety of research and development is performed on the SOI wafer to realize a substrate for next-generation devices.
Examples of the method for manufacturing the SOI wafers typically include a so-called wafer bonding technique and a SIMOX (Separation by Implanted Oxygen) technique. In the wafer bonding technique, oxide films are formed in either one or both of two wafers, and then the two wafers are bonded with the oxide film disposed therebetween so as to manufacture the SOI wafer.
On the other hand, in the SIMOX technique, oxygen ions are implanted into a silicon single crystal wafer, and then the silicon single crystal wafer is subjected to an annealing treatment in an atmosphere of argon or oxygen gas; and thereby, in an oxygen implanted region where supersaturated amount of oxygen is included, silicon atoms are oxidized and the oxygen implanted region is converted into the BOX layer. As a result, it is possible to obtain a SIMOX wafer having a structure where the surface is covered by the SOI layer, and the BOX layer is buried in the inside of the silicon single crystal wafer. Such a SIMOX technique does not need grinding and polishing processes which are needed in the wafer bonding technique; and therefore, the SIMOX technique has the advantage of being able to manufacture the SIMOX wafer through relatively simple processes.
In the case where such a SIMOX wafer is used for manufacturing a system LSI including a logic section, a memory section and the like, the SIMOX wafer is used in which the BOX layer is partially formed such that the BOX layer is formed only in a portion where the logic section is to be formed, and the BOX layer is not formed in a portion where the memory section is to be formed (referred to as “bulk portion”). By using such a SIMOX wafer, it is possible to simplify the LSI manufacturing process which is a post-process.
As the method for manufacturing the SIMOX wafer in which the BOX layer is partially provided as described above, a method has been known which includes forming a mask layer on one surface side of the silicon single crystal wafer such that the mask layer covers the bulk portion and an opening section is provided in a region where the BOX layer is to be formed. In accordance with this method, in the process of implanting oxygen ions, an oxygen implantation region is formed only in a region corresponding to the opening section of the mask layer, and the oxygen implantation is prohibited in the bulk portion due to the barrier effect of the mask layer. Thereafter, only the oxygen implantation region corresponding to the opening section of the mask layer is converted into the BOX layer by the annealing treatment, and thereby, the SIMOX wafer can be obtained in which the BOX layer is partially formed in a predetermined pattern.
In a SIMOX wafer in which the BOX layer is partially formed, there is a problem that the surface flatness is poor. In the oxygen implantation region into which oxygen ions are implanted, the volume increases at the time the silicon is oxidized by the annealing treatment to form the BOX layer. For example, the volume increases by approximately 2.2 times in accordance with an oxidation reaction of Si+O2=SiO2. Thereby, a step difference occurs in the surface of the wafer between the bulk portion and the portion in which the BOX layer is formed. By a simple calculation, with regard to the surface of the portion in which the BOX layer is formed, the surface height after oxidation becomes higher than that before oxidation, and the height difference (step difference) equals to the height of about 55% of the film thickness of the BOX layer. Such a step difference causes overlay issues when a circuit is formed by photolithography in a post-process.
As a method for reducing surface bumps between the BOX layer forming portion and the bulk portion of the SIMOX wafer, for example, Japanese Unexamined Patent Application Publication No. 2004-193185 discloses a method for manufacturing the SOI substrate includes a process of implanting oxygen ions into an inside of a silicon single crystal wafer, and a process of performing an annealing treatment so as to form a BOX layer, wherein the method further includes a process of thinning a first oxide film of a mask layer to a thickness equivalent to an increased amount of thickness to be increased by conversion into the BOX layer between the process of implanting oxygen ions and the process of forming the BOX layer.
In addition, for example, Japanese Unexamined Patent Application Publication No. 2007-142134 or Japanese Unexamined Patent Application Publication No. 2005-268511 discloses a method for manufacturing the SOI substrate which includes a process of etching a BOX layer forming portion (a portion where a BOX layer is to be formed) by dry etching so as to provide a step difference in advance between the BOX layer forming portion and a bulk portion, before implanting oxygen ions into the BOX layer forming portion, wherein the step difference (difference in height) is equivalent to an increased amount of thickness to be increased by conversion into the BOX layer.
Moreover, for example, Japanese Unexamined Patent Application Publication No. 2007-142135 discloses a method for manufacturing the SOI substrate which includes a process of forming a thermally-oxidized film by thermal oxidation and dry etching so as to provide a step difference in advance between a BOX layer forming portion and a bulk portion, before implanting oxygen ions into the BOX layer forming portion, wherein the step difference is equivalent to an increased amount of thickness to be increased by conversion into a BOX layer.
However, for example, in the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 2004-193185, a silicon oxide film is used as the mask layer, and when the mask layer of the silicon oxide film (mask oxide film) is wet-etched to a predetermined thickness, it has been difficult to control the film thickness of the mask oxide film in good uniformity and reproducibility in a wafer and between the wafers. In addition, since the wet etching is an isotropic etching, there has occurred a phenomenon that the side along the thickness direction of the mask oxide film is also etched, and thereby, the width of the mask oxide film or the area of the mask oxide film becomes small when seen in a plan view. As a result, it is likely that the mask oxide film does not cover a desired region, and thus the BOX layer is exposed to the surface, and when the mask oxide film is removed, the exposed portion of the BOX layer is removed; and thereby, there is a fear of forming a cavity. Therefore, there has been a need as well to solve such a phenomenon.
In addition, for example, in the manufacturing methods disclosed in Japanese Unexamined Patent Application Publication Nos. 2007-142134 and 2005-268511, it is difficult to satisfactorily maintain the flatness of the whole surface of the wafer by the dry etching, and it is also difficult to control the film thickness in good reproducibility. In addition, by using the dry etching, physical damage remains in the surface of the wafer, and this also becomes the cause of deteriorating the characteristics of a device.
Moreover, for example, in the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 2007-142135, there has been a problem that when the mask oxide film is formed, a region just below this mask oxide film is also oxidized; and thereby, the size cannot be constantly maintained.
The present invention is devised to solve the above-mentioned problems, and the present invention aims to provide a method for manufacturing a SIMOX substrate in which a BOX layer is partially formed, which is capable of satisfactorily maintaining the surface flatness through simple processes with good reproducibility. In addition, the present invention also aims to provide a SIMOX substrate in which the BOX layer is partially formed, which satisfactorily maintains the surface flatness, and has few surface defects.